Conventionally, as a method for producing crystals, a method in which a seed crystal is contacted with the surface of a previously liquefied raw material and crystals are then grown using the seed crystal as nuclei by decreasing a temperature of the liquefied raw material has been known. As such a method, “TSSG (Top-Seeded-Solution-Growth) method” (e.g., see Patent Document 1) in which crystals are grown from a solution and “crystal pulling method” (e.g., see Patent Document 2) in which crystals are grown from a melt have been known. It is necessary for both methods to regulate the temperature of the liquefied raw material in the range of 0.1 to several tens of degrees in addition to a constant cooling rate in order to control a crystal diameter, i.e., an amount of crystal growth. There has been a first problem in that the growth rate in the crystal growth is changed depending on sites by this temperature regulation and consequently crystal quality of the crystals produces variation.
As methods for solving the first problem, a Vertical Bridgman Method (e.g., see Patent Document 3) and a vertical gradient freeze method (e.g., see Patent Document 4) have been known. In the Vertical Bridgman Method, a crucible is vertically placed to give a temperature gradient. In a temperature distribution, a lower portion of the crucible is made a lower temperature area than a crystallization temperature and an upper portion of the crucible is made a higher temperature area than the crystallization temperature by regulating output power of heating heaters. By keeping the output power of the heating heaters constant, the crystals are grown by moving the crucible to the lower temperature area to cool. Meanwhile, in the vertical gradient freeze method, the crucible is held vertically. In a temperature distribution, a lower portion of the crucible is made a lower temperature area than a crystallization temperature and an upper portion of the crucible is made a higher temperature area than the crystallization temperature by regulating output power of the heating heaters. By keeping this temperature gradient, the crystals are grown from the lower portion of the crucible by changing the output power of the heating heaters.
With reference to FIG. 1, the method for producing the crystals by the conventional Vertical Bridgman Method will be described. A raw material is filled in a crucible 1, and made into a liquefied raw material 2 by heating and liquefying. A crystal-producing furnace has the temperature distribution 5 in which the lower portion of the crucible 1 is a lower temperature area than the crystallization temperature and the upper portion of the crucible 1 is a higher temperature area than the crystallization temperature. By keeping the output power of heating heaters constant, the liquefied raw material 2 is cooled by moving the crucible 1 at a constant speed to the lower temperature area, i.e., the lower portion. Crystals 3 which have reached the crystallization temperature are grown to crystals using seed crystal 4 as nuclei.
With reference to FIG. 2, the method for producing the crystals by the conventional vertical gradient freeze method will be described. A raw material is filled in a crucible 1, and made into a liquefied raw material 2 by heating and liquefying. A crystal-producing furnace has the temperature distribution 5 in which the lower portion of the crucible 1 is a lower temperature area than the crystallization temperature and the upper portion of the crucible 1 is a higher temperature area than the crystallization temperature. By fixing the position of the crucible 1 in the furnace and keeping the temperature gradient shown in the FIGURE, the temperature of the crucible 1 is lowered at a constant rate by changing the output power of the heating heaters. Crystals 3 which have reached the crystallization temperature are grown to crystals using seed crystal 4 as nuclei by changing the temperature distribution.
In the conventional Vertical Bridgman Method and the vertical gradient freeze method, since a shape of the crystal is defined by a shape of the crucible, the temperature regulation for controlling the amount of crystal growth is not required. Therefore, the crystals can be grown by keeping a constant growth rate, and the variation of the crystal quality can be inhibited. That is, the first problem can be solved.
However, when the crystal in which a dopant is added such as In doped GaAs crystal is grown, since a segregation coefficient is not 1, the dopant at a concentration previously added is not incorporated in the crystal by keeping the concentration. Due to this phenomenon, the concentration of the dopant in the raw material is changed as the crystal is grown, and the concentration of the dopant in the crystal is also changed. When a solid solution crystal is grown, a composition of the crystal is changed when crystallized because a liquefied raw material composition and a crystal composition are different. Therefore, in both cases, there has been a second problem in that crystals having a constant composition cannot be produced.
The case of producing solid solution crystals in which the compositions are changed because the liquefied raw material composition and the crystal composition are different when grown by the Vertical Bridgman Method or the vertical gradient freeze method will be described.
A phase diagram of a solid solution crystal composed of ABxC1-x is shown in FIG. 3. Three components, A, B and C may be composed of multiple elements. In the solid solution crystal, ABxC1-x, a liquidus line 6 and a solidus line 7 are generally dissociated. When the composition of the liquefied raw material at point a of the liquidus line 6 is used, the crystal having the composition at point b of the solidus line 7 is grown. The component B is abundantly incorporated in the solid phase, and thus, the component B in the liquid phase is reduced. Thus, in proportion to progress of the crystal growth, the composition of the liquid phase is changed from the point a to point c along the liquidus line 6, and the composition of the grown crystal is also changed to the point d along the solidus line 7.
Therefore, in the produced crystal, the composition is gradually changed from the point b to the point dover a growth direction. In accordance with FIG. 3, the composition in the growth direction of one crystal is changed from AB0.8C0.2 to AB0.4C0.6. When it is wanted to acquire the desired composition from the grown crystal, the desired composition is obtained from only a part of the crystal and productivity is remarkably low. Thus, as shown in FIG. 4, a method in which a resupply raw material 9 is additionally supplied from a raw material supply apparatus 8 during the crystal growth to inhibit the composition change has been attempted (e.g., see Patent Document 2, 5 and 6).
However, in this method, due to the additional supply of the resupply raw material 9, a third problem occurs in which a process yield is reduced due to deterioration of crystal quality and frequent occurrence of polycrystallization. In order to grow the crystals with high quality and good yield, it is necessary to grow the crystals by performing a soaking treatment in which the liquefied raw material is thoroughly decomposed at a temperature (referred to as a soaking temperature) of 20 to 100° C. higher than the crystallization temperature. When the crystals are grown without performing the soaking treatment, the crystal quality is deteriorated and the polycrystallization occurs. When the resupply raw material is supplied, it is also desirable to supply after giving the soaking treatment. However, in the conventional method, it is not possible to supply the resupply raw material after being given the soaking treatment, and the third problem described above has occurred.
Since the resupply raw material 9 is supplied in a powder or a liquid at a temperature close to the crystallization temperature, the temperature of the liquid raw material 2 is changed and the growth rate of the crystal is changed. Due to this variation in the growth rate, there has been a fourth problem in that the crystal quality produces the variation depending on grown parts.
When the crystals are grown from the melt, an overheating treatment in which the raw material is decomposed at a significantly higher temperature than the crystallization temperature is performed in place of the soaking treatment. When the resupply raw material is supplied without performing the overheating treatment, the third and fourth problems described above occur.
Patent Document 1: U.S. Pat. No. 5,785,898 Specification    Patent Document 2: U.S. Pat. No. 5,290,395 Specification    Patent Document 3: U.S. Pat. No. 5,342,475 Specification    Patent Document 4: U.S. Pat. No. 4,404,172 Specification    Patent Document 5: U.S. Pat. No. 5,788,764 Specification    Patent Document 6: U.S. Pat. No. 6,673,330 Specification